E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
According to Scandinavian tradition, this bird hovered over the cross of our Lord, crying Svala! svala! (Console! console!) whence it was called svalow (the bird of consolation). (See CHRISTIAN TRADITIONS.)
The swallow is said to bring home from the sea-shore a stone which gives sight to her fledglings.
Seeking with eager eyes that wondrous stone which the swallow
Brings from the shore of the sea to restore the sight of its fledglings.
Longfellow: Evangeline, part i.
It is lucky for a swallow to build about ones house. This is a Roman superstition. Ælian says that the swallow was sacred to the Penats or household gods, and therefore to injure one would be to bring wrath upon your own house.
It is unlucky to kill a swallow.
Perhaps you failed in your foreseeing skill,
For swallows are unlucky birds to kill.
Dryden: Hind and Panther, part iii.
One swallow does not make spring. You are not to suppose winter is past because you have seen a swallow; nor that the troubles of life are over because you have surmounted one difficulty.